Life history of the Indo‐Pacific humpback dolphin in the Pearl River Estuary, southern China

Abstract

We studied life history characteristics of the Hong Kong/Pearl River Estuary population of Indo‐Pacific humpback dolphins (Sousa chinensis), based on data from 120 specimens stranded between 1995 and 2009, 40 individuals biopsied at sea, and a long‐term (14+ yr) photo‐identification study. Ages were determined for 112 specimens by thin‐sectioning teeth and counting growth layer groups. Estimated length at birth was 101 cm. Longevity was at least 38 yr, and there was little difference in growth patterns of males and females. Growth was described by a Bayesian two‐phase Gompertz model; asymptotic length was reached at 249 cm. The tooth pulp cavity filled at an average of 18.5 yr of age. Physical maturity was reached at between 14 and 17 yr of age, apparently a few years after attainment of sexual maturity. Maximum lengths and weights of about 268 cm and 240 kg were attained. Females appear to lose all their spots by 30 yr, although males may retain some spotting throughout life. Calving occurred throughout the year, with a broad peak from March to June. Of 60 females monitored at sea for >14 yr of the study, none were documented to have more than three calves, suggestive of low reproductive output or low calf survival.

Diagnosability and description of a new subspecies of Indo-Pacific humpback dolphin, Sousa chinensis (Osbeck, 1765), from the Taiwan Strait

“…This subspecies has complex pigmentation with dramatic developmental changes (see Jefferson et al 2012;Jefferson and Rosenbaum 2014). It is medium to dark grey at birth and then transitions with age through various degrees of light and dark spotting while the base layer colour also changes from being primarily grey to white.…”

Possible age-related hearing loss (presbycusis) and corresponding change in echolocation parameters in a stranded Indo-Pacific humpback dolphin

“…Analysis of the teeth, particularly the wear on them, and the skin colour based on the life history information of the Indo-Pacific humpback dolphin (Jefferson et al, 2012) suggested that the dolphin was around 40years old at the time of the study. A hearing test has been deemed an important part of the health assessment of stranded dolphins.…”

Impacts of fisheries on the Critically Endangered humpback dolphin Sousa chinensis population in the eastern Taiwan Strait

“…There is very little information on contaminant levels in ETS Sousa, but high levels of persistent organic pollutants in PRE Sousa are considered as risks to their health (Minh et al 1999, Parsons 2004, Leung et al 2005, Jefferson et al 2006. High rates of neonatal mortality in the PRE Sousa may be caused by exposure of breeding females to environmental contaminants (Jefferson et al 2012). …”

“…Matrilineal social structure has not been conclusively demonstrated for humpback dolphins, but post-reproductive female longevity (a hallmark of matrilineal societies; McAuliffe & Whitehead 2005) is suspected in both the PRE (Jefferson et al 2012) and ETS populations (Dungan 2011). Breeding females appear to have important social network positions in ETS Sousa.…”